1. Field of the Invention
The present invention relates to a process for continuously producing a polyolefin material having a high strength and a high modulus by continuously compression-molding a polyolefin powder at a temperature lower than the melting point and then rolling and stretching the resultant compression-molded polyolefin.
2. Description of the Related Art
So-called ultra-high-molecular-weight polyolefins having a significantly high molecular weight have excellent impact resistance, excellent abrasion resistance, self-lubricating property, etc. and are an engineering plastic having a number of features. They can be made into a molded product of high strength and high modulus if they can be oriented to a high degree by an easy method. Therefore, development of such an easy method is highly expected.
The present inventors previously provided, in Japanese Patent Application Kokai (Laid-Open) Nos. 41512/1988 and 66207/1988 [corresponding to EP 253513 Al (1988)], a process for producing a polyolefin material of high strength and high modulus by batchwise compression-molding an ultra-high-molecular-weight polyolefin powder at a temperature lower than the melting point of the powder without melting it or dissolving it in any solvent, and then rolling and stretching the resultant compression-molded polyolefin.
In order to improve the productivity of the above batchwise process, the present inventors further proposed a process comprising feeding a polyolefin powder between a pair of upper and lower endless belts facing each other, conveying the polyolefin powder between the endless belts under compression to continuously compression-mold the polyolefin powder, the compression being effected by a pressing means comprising pressing platens and corresponding sets of rollers, the pressing platens being arranged within the endless belts, said sets of rollers being arranged between the pressing platens and the endless belts, the rollers of each set being connected in a series and each arranged rotatably, and then rolling and stretching the resultant compression-molded polyolefin [corresponding to EP 0374785 Al (1990)].
In order to carry out the above process more efficiently, the present inventors and the co-worker provided a process comprising processing a polyethylene powder by mixing it with a liquid organic compound having a boiling point not lower than the melting point of the polyethylene [EP 0396 061 Al (1990)] and a process comprising concurrently processing an ultra-high-molecular-weight polyethylene powder and an olefinic polymer having a molecular weight lower than that of the polyethylene powder [corresponding to EP 0410384 A2 (1991)].
The "pressing means comprising pressing platens and corresponding sets of rollers, the pressing platens being arranged within the endless belts, said sets of rollers being arranged between the pressing platens and the endless belts, the rollers of each set being connected in a series and each arranged rotatably" employed in the continuous and efficient process provided by the present inventors, substantially discloses a process wherein each roller of each roller set is fixed at the two ends by chains and these chains are meshed with sprockets arranged at the both ends of the associated platen to allow the set of rollers to travel at an appropriate speed corresponding to the running speed of the associated endless belt. It is described in the disclosure that each set of rollers may be arranged fixedly between the endless belt and the associated pressing platen but, in this case, a frictional force is generated by slip between the rollers and the endless belts and between the rollers and the pressing platens, giving rise to the durability problems of the apparatus.